1. Technical Field
The invention relates to data storage systems for data processing systems, and more particularly relates to controllers for disk drives serving a plurality of host main frame computers.
2. Description of the Related Art
Main frame computer systems generally use auxiliary storage devices for mass storage of data. Direct Access Storage Devices (DASD) are one such type of auxiliary storage device. A direct access storage device is one in which access time is effectively independent of the location of data in the storage device. Direct access storage devices may be located externally to a host main frame computer and can serve several computers. Data integrity requires that only one computer use or own a direct access storage device at a time. The system designer must provide for resolution of conflict between the computers for access to the storage device.
One common type of direct access storage device is the magnetic disk unit, which includes a disk drive, magnetic transducers for writing data to and reading data from the disk drive and associated controls for positioning the transducers, among other tasks. For applications such as data base storage, several magnetic disk units may be grouped together, and a storage controller logically positioned between the group and several independent computers. The storage controller handles connection and disconnection between a particular computer and magnetic disk unit. It is to the storage controller that the task of resolution of conflicting demands of host computers for magnetic disk units falls.
The IBM 3990 storage controller is an example of a storage control unit used to handle connections between selected members of a group of magnetic disk units and computers requesting connection. The IBM 3990 Model 3 type controller, which is available from IBM Corporation, can handle up to sixteen channels from host computers and up to sixty-four magnetic storage units. Within the storage controller are two multipath storage directors and four storage paths, two of which are associated with each multipath storage director. Each multipath storage director may be connected to up to eight incoming channels from host computers, for a total of sixteen channels for the storage controller as a whole. Each multipath storage director is connected to two storage paths, thus functioning as an 8.times.2 switch. Channels are physical links between the storage controller and the host computers. Usually, a host computer has two or four channels, half being physically connected to the one multipath storage director and the other half being connected to the other multipath storage director.
Each storage path is connected to all of the available magnetic disk units. Upon indication that a disk is ready, the controller can secure any one of a plurality of the channels and storage paths back to a host to establish a data path. It is a feature of the IBM 3990 storage controller that an input/output transaction between a host computer and a magnetic disk unit may be broken into two separate connections, which may be handled along distinct channels and storage paths. A request need not be responded to over the same channel on which it was received. This feature increases throughput through the storage controller. While each storage path is in effect a stand alone control unit based upon its own microprocessor, the storage paths share control arrays in a common memory space for synchronization functions relating to handling connections, disconnections and reconnections relating to a transaction.
Several manufacturers, including IBM, Amdahl and Hitachi, supply computers of various models connectable to the IBM 3990 storage controller. These computers differ in operating speeds and in time required to respond to signals from storage controllers. The channels used for connection of the computers to the storage controller may also differ. One type of channel has parallel copper wire paths for transmission of electrical signals. This channel type is used at distances of up to about 125 meters. Serial optical cables are used to connect computers and storage controllers at distances out to 15 kilometers.
A computer requesting connection to a magnetic storage unit that is already in use is given a busy signal and refused connection by the storage controller. Upon the storage unit being released, an Owed Device End (ODE) signal has been sent by the storage controller to all channels that received a device busy signal during the period the device was taken. Similarly, all storage paths in a storage cluster may be occupied by connections between storage units and computers. A computer requesting a connection under these circumstances is given a control unit busy signal. Upon release of one of the connections, all computers having received the busy signal are provided a Control Unit End (CUE) signal. The ODE signal and the CUE signal are akin to starting guns in a race among the computers to take control of the storage director or storage device. Control is granted on a first come, first served basis.
The race is not limited to the entities receiving the CUE and ODE signals. Any of the computers attached to the storage controller may receive control of a storage path or storage device for the asking by being the first to have its request received. Thus the channel which just released a storage path may make a request to regain control of the path.
The conflict resolution regime is not a race among equals. Computers which are faster can respond to the CUE and ODE signals more quickly. Where the channel between computer and storage controller is physically short, the transmission time for the CUE and ODE signals from the storage controller to the computer and the transmission time for a resulting Start Input/Output (SI/O) signal from the computer to the storage controller is commensurately short. Computers advantaged by a short channel may beat far removed computers to the storage controller with a SI/O signal even though they have not received a CUE or ODE signal. The problem caused by this system is that during periods of high demand for access to a data base stored on a group of magnetic storage devices, the slow or more distant computers may be locked out of access to the storage devices by computers which are faster or which have shorter transmission link times to a storage controller.
It is an object of the invention to provide a method of allocating access among computers to storage paths in a storage controller and to direct access storage devices.
It is another object of the invention to prevent lock out of computers from access to either storage paths or to direct access storage devices.
It is still another object of the invention to identify failing host computers.